Microscopes, as a rule, have a focus drive which is manually operated by means of two coaxially arranged knobs, for vertical displacement of either the microscope stage or the lens-mounting turret. One of these actuating knobs serves for rough adjustment; and the other, via gearing of strong step-down ratio, serves for fine adjustment. These knobs are frequently arranged laterally on both the left-hand side and the right-hand side of the microscope stand.
Microscopes are also known to have a focus drive which can be motor-driven. In the case of microscopes for routine examinations, as for example, examination of products of the semiconductor industry, a so-called "autofocus" is frequently provided with a motor drive which automatically brings the focal plane of the microscope optical system onto the surface of the object to be examined. It is customary to connect the motor on the shaft of the focus drive, in place of the actuating knobs on one side of the microscope; in such case, the remaining actuating knobs on the other side of the microscope serve for manual focusing.
This "autofocus" arrangement is, however, frequently unfavorable. Thus, for example in the semiconductor industry, when inspecting wafers or masks, it is important that the operator shall not come too close to the specimen objects on the microscope stage, in order to reduce as much as possible the danger of dust-particle contamination of the specimen. Furthermore, at inspection work stations, the microscope is generally incorporated in a handling system for the automatic feeding, loading and transfer of specimens in and out of the microscope-viewing region, so that operating means on the microscope stand are accessible only with difficulty, if at all.
Federal Republic of Germany OS No. 3,410,201 discloses a microscope with a motor-driven focus drive in which the motor is coupled, via a magnetically shiftable clutch and spur gearing, to the shaft which interconnects the fine-adjustment operating knobs on the respective lateral sides of the microscope. The base of the microscope is provided with additional operating means in the form of a switch by which the drive motor for the focus drive can be operated for a rapid (rough) focusing adjustment. The speed of focus adjustment in this case depends on the power of the lens currently in operative position. This known microscope also has the disadvantages mentioned above since all operating means, namely, the adjustment knobs and the switch, are arranged on the microscope stand itself.